A typically known turbocharger compresses gas such as air to increase its density, and supplies the gas as a combustion gas to a combustion chamber of an engine (internal combustion engine). A turbocharger includes a rotor shaft (rotational shaft), and a turbine and a compressor disposed on either end of the rotor shaft. The turbocharger converts energy of exhaust gas discharged from a combustion chamber of an internal combustion engine to an exhaust channel into rotational energy with the turbine to drive the compressor, and compresses gas such as air to supply a combustion gas to the combustion chamber. With the turbocharger provided for the engine, it is possible to achieve the same engine output with less fuel, which makes it possible to improve the engine output and to reduce the amount of fuel consumption. On the other hand, the turbine is driven depending on the energy of exhaust gas, and thus the advantageous effect of the turbocharger is small during low-load operation of the engine when the flow rate of exhaust gas is small. For instance, ships occasionally sail in a low-speed operating range to cut the amount of fuel consumption (fuel cost), where the engine is at low load and the flow rate of exhaust gas is small, resulting in a remarkable shortage in the capacity of the turbocharger.
In view of this, developed is an electric-assisted turbocharger including a motor (electric motor) capable of rotating a rotor shaft independently from exhaust gas (see Patent Document 1, for instance). For an engine equipped with an electric-assisted turbocharger, shortage in its capacity is made up by increasing the rotation speed of the rotor shaft with the motor during low-load operation. On the other hand, while the engine is in a high-load operating range, energy of exhaust gas is sufficient and thus the motor is stopped. Further, as a type of electric-assisted turbocharger, known is a hybrid turbocharger, which supplies compressed air similarly to an electric-assisted turbocharger but also recovers excessive energy of exhaust gas as electric power during high-load operation of the above engine.
As a structure of such an electric-assisted turbocharger, known is a motor over-hung structure with a small motor mounted to a shaft-extended portion extended from a compressor-side end portion of a rotor shaft (see Patent Document 1). With the motor over-hung structure, two bearings are normally provided to support the rotor shaft, which can also support the weight of the small motor sufficiently. Thus, it is unnecessary to provide another bearing dedicated to supporting the motor. However, when an electric-assisted turbocharger has a heavy object (the motor) disposed on a tip of the rotor shaft positioned outside the bearings, the heavy object is likely to cause shaft vibration, which may generate undesirable noises. For instance, Patent Document 2 discloses a method for reducing noise and vibration of an electric turbocharger (supercharger), by providing an elastic material between a turbocharger, an electric motor, and a casing that houses an inverter, and also applying a sound-absorbing material to ducts. Vibration noises are mainly caused by propagation of shaft vibration of the rotor shaft to the casing or the like, and thus Patent Document 2 can reduce such noises by blocking the propagation of vibration with the elastic member.